Thorin's long-windedness is a very, let's say, dwarven nobility thing. There is lots of pride, ceremony, attempts at high-mindedness, and bad poetics, that are associated with that. He is a royal windbag, in other words.

The legalese of the contract is a different thing altogether. It is a joke on Bilbo, not a reflection of Thorin's character. Bilbo is a sort of Edwardian bourgeois, who likes things written down nice and neat, and Thorin is poking fun at this "grocer-like" attribute of Bilbo by writing up this faux-contract.

In the films, the contract will be a reflection of the dwarves. And frankly, that's a misinterpretation of who they are. The joke should be on Bilbo, not the dwarves.